Wireless direction finding system



1935. H. M. DOWSETT ET AL 2,018,349

WIRELESS DIRECTION FINDING SYSTEM Filed June 22, 1934 INVENTOR weer 4/ flan 5577' za/zs a/mLm/qse.

ATTORNEY Patented Oct. 22, 1935 UNITED STATES PATENT OFFICE v 2,018,349 WIRELESS DHtECTION FINDING SYSTEM Harry Melville Dowsett, Winchmore Hill, London, and Louis Edward Quintrell Walker, Maida Vale, London, England, assignors to Radio Corporation of America, a corporation of Delaware 7 Application June 22, 1934, Serial No. 731,814 In Great Britain May 6, 1933 8 Claims.

atively unskilled persons.

In known wireless direction finding systems as at present in common use, hearings are usually obtained by one of two difierent broad methods. Either the receiving station is provided with a rotatable frame aerial or a pair of fixed frames and a radio-goniometer the operator moving the rotating frame or the radio-goniometer searchcoil until a maximum audible signal or a minimum audible signal (as the case may be) is received; or transmitting stations of the rotating beacon type are employed the emission for these stations being modulated with signals which will indicate at a receiver (which need not be a directional receiver) the direction of the transmitting station being received. As a rule in the latter type of system a characteristic signal is emitted from the transmitting station when the beam thereof passes through true North and the beam is rotated at a known predetermined rate so that the receiving station can tell the direction of the transmitter by observing the interval of time between the emission of the characteristic North signal and the instant when the rotating beam passes through the receiver. Whichever method of direction finding is employed the signals utilized are as a rule Morse code signals and a considerable amount of skill and judgment is required on the part of the wireless operators (particularly at receiving stations).

Our co-pending U. S. application Ser. No. 672,552, filed May 24, 1933 describes an arrangement wherein the amount of skill which is necessary for obtaining direction finding bearings is reduced by utilizing a television receiving set at a receiving station in a directional or, bearing finding radio system and a corresponding television transmitting set at the transmitting station, the required indication of the direction of received signals and an indication identifying the beacon station being received, or both, being given by means of a television picture which is transmitted instead of the hitherto customary audible signal and the nature of which directly indicates at the receiver the direction of the station being received.

A form of transmitting station described in our United States application No. 672,552 May 24, 1933, comprises means for emitting a directional radio beam which is continuously rotated about said station, means for emitting further radio energy omni-directionally, means for modulating the omni-directionally transmitted energy with television picture signals corresponding to a picture giving an indication of the instantaneous direction of the directional beam together with an indication identifying the station, and means for modulating the directional beam with further signals adapted-to produce in a television receiver obtaining signals from both transmitting aerial systems, a picture corresponding to that transmitted omni-directionally and upon which is superimposed an indicatingline or bar.

As will be seen later the present invention provides an improved system wherein the type of picture to be observed in order to obtain a bearing is one in which the indicating marking i. e. a black bar, is at its maximum when the bearing indication is given, so that a positive as distinct from a negative indication is given. Further the present invention provides an improved system having the important practical advantage, as in the case of the arrangement described in the complete specification above referred to, both omnidirectional and directional signals are transmitted from a transmitting station, these two types of transmitted radiation do not occur simultaneously, and therefore the one form of radiation does not interfere with the other as regards the polar diagrams produced. This advantage is in practice of considerable importance.

According to this invention a transmitting station for use in a wireless direction or bearing finding system wherein television signals corresponding to a picture giving an indication identifying the said station are transmitted, the receiving station or stations in the system being provided with corresponding television receiving sets, comprises i a directional transmitting aerial system, means for causing the directivity of said aerial system to be continuously rotated about the station, means for emitting radio energy omni-directionally, means for modulating the omni-directionally transmitted energy with television picture signals obtained by scanning a picture indicative of the instantaneous direction of the directional system together with an indication identifying the station, means for interrupting said omni-directional radiation for a predetermined short interval for each scanning operation, means for radiating energy from said directional aerial sys tem during the intervals when the omni-directional radiation is interrupted, and means for modulating the directionally transmitted'energy with further, arbitrarily chosen, signals The invention is illustrated in the accompanying drawing in which Figs. 1-4 illustrate diagrammatically a preferred form of transmitting station in accordance with the said invention.

Referring first to Figure 1 which shows diagrammatically the general arrangement oil the whole transmitting station the said station comprises "a continuously rotating frame aerial I which is rotated by means of a motor (not shown) in any convenient manner. Driven with the rotating shaft which rotates the frame aerial is a drum 2 carrying a suitable .scale, the-scale moving behind an indicating window (not shown).

The drum is so positioned with respect to the window that the scale reading within the window will correspond to the orientation of the frame aerial. Within the window and immediately beneath the scale, for example forming part of the window, is a fixed indication 3,, e. g. a plurality of printed letters, identifying the transmitting station. Light is projected upon'the rotating scale within the window area and a picture corresponding to the said area and includ ing the portion :of the scalewithinrthe window and the identification letters or the =like at 3, is scanned bymeans of a suitable scanning mechanism "4 which is such that the whole picture, minus a predetermined number of scanning lines, say two scanning lines, is scanned. For=example, light reflected from the picture area may pass through a. scanning disc or socalled Nipkow disc as illustrated in Figure 1 :on to a photoelectric cell 5 (Figure '1) the Nipkow'disc having, however, an incomplete single turn spiral =of apertures. For example as shown in Figure 2 two adjacent apertures, say mid-way of the spiral of apertures (between -0 and :11) may .be omitted. The light passing through the disc on to the photo-electric cell E-gives rise to television signals which, after suitableamplification in known manner at 6 are transmitted froman omni-directional aerial system 1 as modulations upon 'a carrier wave. It will be appreciated that the picture transmitted from this aerial system will be the whole picture scanned minus two scanning lines; assuming horizontal scanning, two horizontal scanning lines will .be omitted.

Also'driven by the shaft which drives the scanning disc is a commutator or other convenient switch control device S which is arranged to shut down radiation from the omni-di-rectional aerial during the short period of timeper scanning operation corresponding to the omitted scanning lines and during this short period time signals are transmited from the frame aerial. A convenient way of doing this is (as illustrated in Figure 2) to provide the scanning disc with two further apertures a b arranged on :a circle of considerabiy smaller radius than the mean radius of the spiral of scanning apertures,thesesaid further apertures being diametrically opposite to that per ion (c to d) of the disc from which the two scanning apertures have been omitted. A source a of-light is provided to project light through the two further apertures a :b on to a second photo-electric cell 9 the :output of which is employed to modulate at ii! the signals transmitted from the frame aerial. Means, which may consist of an additional commutator or other when radiation is prevented from the omni-directional aerial. Alternatively purely electrical means, i. e. means not including -mechanically moving parts, may be employed for preventing for transmission from both aerial systems. 5

It will be seen that with the arrangement above described and illustrated the signals transmitted from the frame aerial l occur in the gaps in the picture signals which are transmitted omni-dire'ctionally from 1 so that a receiving station will 10 normally receive a picture as shown in Figure 3 and which has a small gap in it. When, however, the signal intensity from the rotating frame aerial becomes a minimum, 1. e. when the directionof the frame aerial is at right angles to a line join- 15 ing the transmitting station to the receiving station, ,a black bar will fill the aforementioned gap ass-hown in Figure 4 and the occurrence of this black barmat maximum intensity will give a direct indication of the direction of the transmitter, 20 for rat the instant when the black bar reaches this maznmum the scale reading in the picture will be that-giving the bearing.

Having now particularly described and ascentained the nature of our said invention and in 25 what-manner the same is to be performed we declare that what we-claim is:-

1. In a transmitting station for use in a wireless direction finding system wherein television :signals corresponding to a picture giving an indi- 30 cation identifying the said station are transmitted,

"a directional aerial, a non-directional aerial, means for causing-the directivity of said directional aerial to be continuously rotated, means for energizing said non-directional aerial, means 35 for modulating the energy supplied by said last named means with energy characteristic of television picture signals obtained byscanning a picture indicative of the instantaneous directivity of the directional system and with energy identify- 40 ing the station, means for interrupting said nondirectionalradiation for a predetermined short interval during each scanning operation, means for energizing said directional aerial system during the intervals when the non-directional radia- 45 .tion is interrupted,- and means for modulating the "energy-supplied to said directional aerial.

, an indication of the identity of the station, means for deriving picture signals from said scanning operation and transmitting them from the nondirectional aerial, means for preventing radiation 60 from.said'non-directional aerial at predetermined regular intervals so that a given portion of the picture is omitted and means for transmitting energy from the directional aerial during the periods in which transmission from the non-direc- 65 tional aerial is cut off.

3. A transmitter as recited in claim 2 wherein the periods of interruption of the non-directional transmission is such as to leave a horizontal bar omitted from the picture transmitted. 70

- 4. A station .as recited in claim 1 in which the television scanning means is also provided with scanning means consists of a Nipkow disc or its equivalent modified by the omission of one or more normally provided apertures and also by the addition of a further aperture or apertures arranged to pass light to a light sensitive means which is connected and arranged to secure transmission from the directional aerial system when activated said further aperture or apertures being so positioned as to pass light for periods corresponding to the periods in which scanning action ceases by reason of the omission of the normally provided scanning aperture or apertures.

6. A station as recited in claim 1 wherein the energy radiated from the directional and nondirectional aerial systems is upon the same wave length and is derived from transmitters which are driven by a common high frequency carrier drive source.

'7. A transmitting station as recited in claim 2 in which the non-directional picture transmission scanning means includes a scanning disc, and in which certain scanning elements on said disc are omitted to omit a predetermined scanning line or series of lines from the picture as scanned.

8. A directional signalling system comprising a non-directional aerial, a directional aerial, a direction indicating scale, station identifying characters, means for rotating said directional aerial and said direction indicating scale in synchronism, a scanning disc, means for rotating said scanning disc, a source of high frequency energy arranged to be impressed on both of said aerials, a source of light and a light sensitive cell cooperating with said scanning disc and scale and said direction indicating characters to A impress onv the high frequency energy supplied to said non-directional aerial modulations characteristic of the position of rotation of said scale and characteristic of said identifying characters, 15

a second source of light and a cell cooperating with said disc for impressing on the high frequency energy supplied to said directional aerial characteristic modulations, and means cooperating with said disc rotating means for alternately zo interrupting the supply of energy to said aerials.

HARRY MELVILLE DOWSETT. LOUIS EDWARD QUIN'I'RELL WALKER.

with said 0 

